It's pretty normal at 15 to have irregular periods- late periods can be caused by stress, or a number of other things. Sometimes even physical activity (like if you started playing/training for a sport more intensely in then in the past- it could be causing the delay. Not knowing anything else about your medical history, I would say it's not anything to worry about and won't prevent you from having children in the future. If you go in for yearly check ups, you can mention it to your doctor just for peace of mind- but unless you have other odd symptoms (like your cramps have become way more intense or something), I wouldn't over think it.
P.S.
I haven't gained or lost any weight, gotten sick, and my periods usually last 3-4 days whenever they happen. Is this normal...?